A display apparatus for displaying a virtual image superposed upon a real world image, which can automatically set the focus of a lens to the virtual image, a virtual world image object having a high importance degree or a real world image object having a high importance degree. The polarizing device disposed between the collection system and the detector. Parameters related to the chemicals being used for surface treatments are measured in real time and on-line, and these measured parameters are used to control the dosage of chemicals into the surface treatment system. in a second, rapid-warming stage.